(1) Field of the Invention
The present invention relates to an electron gun that is structured so as to attain a high degree of resolution over the entire screen, and a color picture tube apparatus that includes the electron gun.
(2) Related Art
Generally, a color picture tube apparatus has an envelope formed from a panel and a funnel joined to the panel, and displays color images by emitting three electron beams from an electron gun disposed in a neck of the funnel, onto a phosphor screen formed opposite a shadow mask on an inner surface of the panel, while scanning horizontally and vertically. The three electron beams are deflected by horizontal and vertical deflection magnetic fields generated by a deflection device mounted to the outside of the funnel.
The magnetic fields generated by the deflection device used in the above color picture tube apparatus generally have a self-convergence structure to focus the three electron beams on the screen, and as a result the horizontal and vertical deflection magnetic fields are distorted into a pin-cushion shape and a barrel shape, respectively. The three electron beams that pass through the deflection magnetic fields are thus subject to divergent action in the horizontal direction and convergent action in the vertical direction.
When the electron beam trajectory is lengthened due to an increase in the deflection angle, astigmatism becomes pronounced because of these self-convergence magnetic fields, particularly in outer areas of the phosphor screen surface, and horizontal resolution is reduced as a result of the electron beam spots becoming a flattened, oblong-shape along a major axis in the horizontal direction when viewed in cross-section. This problem has been accentuated in recent years as panels become flatter and deflection angles increase.
Thus, in order to portray high-resolution images on a phosphor screen, it is first necessary to reduce spot diameter in the horizontal direction with the electron gun.
A known technique that attempts to do this involves applying a voltage (dynamic voltage) to a focus electrode in the electron gun. According to this technique, a voltage that increases as the amount of electron beam deflection increases is applied to a focus electrode positioned closest to and facing a final accelerating electrode, and as a result, the action by the main lens electric field weakens as the deflection angle increases, astigmatism is corrected, and the shape of the beam spot is controlled.
Furthermore, Japanese patent No. 3,040,272 discloses a technique that adjusts the shape and orientation of the electron-beam through-holes of the electrodes and specifies conditions of voltage applied to each electrode so that the convergence power of the main lens is stronger in the horizontal direction than the vertical direction. This lowers the dynamic voltage that is applied to the focus electrode, thus reducing the size of the voltage circuit.
It is known that generally a larger electric field lens aperture reduces spherical aberrations and a smaller spot diameter is obtained. This enables improvement in resolution.
The OLF (over-lapping field) lens disclosed in Japanese examined patent application publication No. 2-18540 is an example of technology that realizes this idea by way of the electrode configuration. This OLF lens consists of three lenses that correspond to the three electron beams R, G and B. These three lenses partially overlap.
FIG. 1 is a cross-sectional diagram of the electrode configuration that forms this OLF lens. As shown in FIG. 1, the main electrodes are constituted by a focus electrode 101 and a final accelerating electrode 102 provided with a gap therebetween in a tube axis direction, and a shield cup 103 connected to final accelerating electrode 102.
The focus electrode 101 and the final accelerating electrode 102 are formed respectively from (i) tubular circumferential electrodes 101A and 102A, each of which has a horizontally wide, flattened tube-shape, and encompasses the three electron beams, and (ii) correction electrode plates 101B and 102B, each of which is set back from the facing edges of the tubular circumferential electrodes, and has three noncircular holes 101B1, 101B2, 101B3 and 102B1, 102B2, 102B3, respectively, opened therein to allow the electron beams to pass through substantially perpendicularly.
These correction electrode plates 101B and 102B generate three main lens electric fields that correspond respectively to the three electron beams.
By providing the correction electrode plates 101B and 102B back from the facing edges of the tubular circumferential electrodes 101A and 102A in the focus electrode 101 and the final accelerating electrode 102, respectively, the high potential of the final accelerating electrode 102 is allowed to incur deep into the focus electrode 101, and the low potential of the focus electrode 101 is allowed to incur deep into the final accelerating electrode 102. As a result, the lens aperture resulting from the main lens electric fields is effectively enlarged, and the spot diameter on the phosphor screen can be reduced.
However, the following two problems arise when the main lens is an OLF lens.
The first problem is that it is difficult make the convergence power of the main lens different in the horizontal and vertical directions.
Since the three lenses in an OLF lens partially overlap and interfere with each other as described above, the OLF lens has an asymmetrical three dimensional structure.
Consequently, a problem with OLF lenses is that the lens design is complicated compared to a conventional electron gun main lens.
Specifically, the convergent actions of the three lenses can be made the same by uniformly arranging the tubular cross-sectional shapes of the final accelerating electrode and the preceding focus electrode and the size of the three electron beam through-holes. However, in an OLF lens, the electric field lenses are determined by a plurality of parameters including the shape of the openings of the tubular circumferential electrodes 101 and 102, the position of the correction electrodes 101B and 102B, and the shape of the electron-beam through-holes 101B1 to 101B3 and 102B1 to 102B3.
When design of the electric field lens is so difficult, it is difficult to make a difference between horizontal and vertical convergence power of the main lens (hereinafter referred to as the “HV differential”), and the design of the HV differential of the main lens becomes limited. As a result, aligning the diameters of the electron beams in the horizontal and the vertical directions difficult, and there is little significance to the increased aperture size of the main lens that has been provided to the improve resolution.
Furthermore, when minimizing the applied dynamic voltage to reduce the size of the voltage circuit as an application of the above-described dynamic voltage technique, it is essential to ensure that the main lens has at least a set HV differential. However, as described above, in an OLF lens design difficulties make it is hard to attain the desired HV differential, and this technique is difficult to realize.
The second problem is the occurrence of mislanding which causes color discrepancy.
This second problem is described with use of FIG. 2. Three electric field lenses 201, 202 and 203 that compose the OLF lens are related to the shape of the opening of the tube-shaped electrodes that generate the OLF lens, the shape of the electron-beam through-holes in the correction electrode, and so on. As shown in FIG. 2, if overlap areas 204 of the three electric field lenses 201, 202 and 203 are increased in order to increase the diameter of the apertures, since the outer electric field lenses 201 and 203 are limited by the shape and so on of the tube-shaped electrodes, the centers of the outer electric field lenses 201 and 203 have to be moved closer to the center of the electric field lens 202, and an interval S between the center of each of the outer electric field lenses 201 and 203 and the center of the center electric field lens 202 decreases. Suppose a design in which the electron beams are pass through the effective center of the electric field lenses so as to be symmetrical on the screen 205, and the phosphor dots are provided at a set interval (pitch) on the screen 205. Here, if the interval S decreases, it is necessary to have an increased gap Q between the screen 205 and the shadow mask 207 in order to converge the three electron beams on the shadow mask 207. However, if the gap Q is increased in this way, geomagnetism between the screen 205 and the shadow mask 207 mis-aligns the trajectories of the electron beams, thus causing color discrepancies, and deterioration in resolution in the outer parts of the screen.